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An on-orbit servicing satellite (OOS) is one what would be use for servicing, refueling, assembly in space.
Would there be any specific orbit or standard orbit or at least most-likely range of orbits for an OOS?
Would there be any particular challenges placing or using them in GEO?
The altitude of a servicing satellite will be dependent on its client, of course. There is no additional challenge to placing the servicing satellite in a given orbit than for any other satellite.
To date, the only servicing satellite launched (not including the space shuttle) is the Mission Extension Vehicle (MEV). It was placed in a super-synchronous orbit by its Proton launch vehicle and will use electric propulsion to reach the geosynchronous graveyard orbit (about 200-300 km above geosynchronous orbit) where it will rendezvous with Intelsat 901. If successful, it will then bring Intelsat 901 back down to the operational geosynchronous orbit.
Would there be any specific orbit or standard orbit or at least most-likely range of orbits for an OOS?
You would most likely need one OOS for each commonly used orbital plane, as altitude changes and rendezvous along the same plane are relatively straightforward from a delta-v budget perspective. Plane changes, on the other hand, require quit a bit of delta-v and thus quite a bit of fuel. Some commonly used "standard orbits" that satellites are typically placed in include: Equatorial, Ecliptic, polar, and polar Sun-synchronous. There are also a number of highly inclined orbits that are popular for earth observation.
In geostationary or geosynchronous orbits, getting around is certainly easier as there are only a small handful of orbital planes used.
Would there be any particular challenges placing or using them in GEO?
High and medium earth orbits are certainly more challenging than LEO, but the majority of those challenges are well understood and should not preclude the development of a functional OOS.
